Traditionally, polyurethanes have been used in a wide range of fields such as plastic materials, fibers, paints or adhesives. Recently, polyurethanes in which a biodegradable and biocompatible polylactic acid-based polyol is used as a raw material are drawing attention in view of environmental issues. Also, polyurethanes using a polylactic acid-based polyol having shape-memory property as a raw material are known (Patent Document 1), and they are investigated for applications in electronic or medical materials. Polylactic acid-based polyols used in manufacturing urethane resins are obtained by ring-opening polymerization of lactides in which compounds having two or more hydroxyl groups are used as an initiator, or by transesterification between a compound having two or more hydroxyl groups and a commercially available polylactic acid resin. When a polylactic acid-based polyol is produced by lactide ring-opening polymerization, lactides are needed for purification because low purity lactides cannot produce high molecular weight polymers. In addition, lactides are expensive. Transesterification has higher productivity than ring-opening polymerization because it can produce polylactic acid-based polyols only by mixing raw materials in solution.
However, polylactic acid-based polyols obtained by transesterification contain oligomers of byproducts from transesterification, and a polymerization catalyst used in synthesizing a polylactic acid-based compound supplied as a raw material for transesterification. These impurities which remain in the polylactic acid-based polyol affect urethane reaction, and thus a purification process is needed. Methods for purification include a reprecipitation method in which a polylactic acid-based polyol is dissolved in a solvent such as chloroform and the resulting solution is added to a poor solvent such as methanol to precipitate impurities, and a reduced pressure method in which oligomers are removed by forming a reduced pressure in a system after an esterification reaction is completed. The reprecipitation method can remove a catalyst as well as oligomers. However, this method has high environmental load and low productivity due to the use of enormous solvents. The reduced pressure method reduces pressure using an apparatus such as a distiller attached to a reactor. Therefore, this method is an easy method for purification having high productivity.
The reduced pressure method can remove oligomers having a low boiling point. However, in this method, polymerization catalysts having a high boiling point remain in polylactic acid-based polyols. Polymerization catalysts also have a catalytic function in a urethanization reaction of polylactic acid-based polyols. For this reason, when synthesizing polyurethanes using polylactic acid-based polyols purified by the reduced pressure method, the urethanization reaction proceeds by the remaining polymerization catalyst. In this case, components supplied for urethanization may not be homogeneously blended, and consequently it is difficult to obtain a uniform molded product. Therefore, as deactivators to form a chelator with a polymerization catalyst in a polylactic acid and deactivate the catalyst, phosphoric acids, citric acids, phenolic compounds, multifunctional amine compounds and the like are known in the art (Patent Documents 2 and 3). However, since these deactivators are acidic or alkaline compounds, they have a problem in that polylactic acids may be degraded or urethanization may be inhibited due to a reaction with isocyanate groups. In particular, acids are reacted with isocyanates to produce carbon dioxides, and consequently a uniform molded product may not be obtained. Additionally, a method of deactivating the remaining catalyst by a hot water treatment was reported (Patent Document 4). However, in this method, water also may cause the degradation of polylactic acids or the formation of bubbles in the same way as acids.
Further, silica compounds are known in the art as a catalyst-adsorbing material (Patent Document 5). In Patent Document 5, a tin catalyst used in the synthesis of a polymer is reacted with silanol groups on the surface of a silica compound to synthesize a solid catalyst previously, and the resulting catalyst is used as a catalyst for urethanization. Herein, since the adsorbed tin catalyst acts as a catalyst, said method is not intended to deactivate a catalytic function. Also, according to Examples of this Patent Document, it takes 3 hours to heat the silica compound and the tin catalyst in synthesizing the solid catalyst. It is difficult to apply such a method for synthesizing a solid catalyst to a polylactic acid-based polyol obtained by transesterification under the same reaction condition due to the risk of degrading a resin. In addition, a tin catalyst adsorbed on the surface of a silica compound has a decreased dispersion in a urethane ingredient, and consequently it is difficult to obtain a uniform molded product.